1. Field of the Invention
The present invention relates to the field of shared point-to-multipoint or multipoint-to-multipoint communication networks and, more specifically, to orthogonal frequency division multiple access communication networks (FDMA or FDD).
The present invention will be described hereafter in relation with an example of application to networks using the electric supply conductors (for example, the mains) as a transmission medium. High-frequency carriers which are modulated to transmit data between two or several devices equipped with modems (modulator-demodulator) and connected to the mains are generally used. Such networks may be used, for example, to connect a microcomputer to its peripherals (printer, scanner, etc.). They may also distribute, inside of a home, an office, or the like, multimedia data coming from a connection to an external access, for example, a satellite antenna, an optical fiber cable, a modem cable, an XDSL modem, etc.
2. Discussion of the Related Art
Conventionally, the standards relative to networks using the supply system as a transmission support provide the combined use of a frequency-division multiple access and of a time-division multiple access. This actually is a carrier detection and collision or anticollision detection multiple access (CSMA/CA). According to these transmission standards, the frequency division multiple access is implemented by using an orthogonal frequency-division multiplexing (OFDM).
This is a well known technique which will be briefly described hereafter. Reference can also be made to literature. For example, article “HomePlug Standard Brings Networking to the Home” by Steve Gardner, Brian Markwalter, and Larry Yonge, published in December 2000 in Communication Systems Design, and incorporated herein by reference, discusses the application of such a multiplexing to networks using electric power cables as a transmission support.
The OFDM waveforms are generated by using inverse Fourier transforms (IFFT) in which the points of the frequency field are formed by complex sets of symbols which modulate each carrier. The result of the inverse Fourier transform is called an OFDM symbol. On the receive side, the data are reconstituted from a direct Fourier transform which converts the OFDM symbol in the frequency field. It should be noted that of this processing remain digital.
Each modem connected to any node of the network must be able to send data to any other node and to receive data coming from any other node.
Since various devices may simultaneously need to transmit information over the shared network, an access control mechanism of the transmission means (MAC) is necessary to avoid collisions which would result in information losses. Further, since different multimedia services or transmission types are likely to use the same electric supply conductors and since these different transmissions most often have distinct constraints in terms of delay, bit error rate, etc., an access priority management mechanism more generally designated as a quality-of-service control (QOS) is generally used.
In a frequency-division multiple access, each network node (that is, each modem connected to the network) is assigned a predefined set of frequencies to transmit data. On the receive side, there exist two solutions. Either a communication control channel is used to indicate which frequencies must be received and demodulated by each device connected to the multipoint-to-multipoint network. Or all receive frequencies are demodulated by all devices and each receive device selects the information intended thereto.
A specificity of networks using the supply conductors as a transmission support is that the network transfer function considerably varies along time (for example, under the effect of the plugging of an electric device, be it or not equipped with a modem), that the transfer function varies from one node to another, and the fact that it is a multiple-path and multiple-reflection network. As a result, the transmissions of a modem in a frequency set risk being randomly polluted, preventing proper reception by another modem.